This invention relates generally to light-emitting diodes and, more particularly, to surface-emitting light-emitting diodes (LEDs). Light-emitting diodes are semiconductor devices operating on a well-known principle whereby light is emitted from a forward-biased junction between selected semiconductor materials. The semiconductor structure of a light-emitting diode is similar to that of a laser diode, except that the light-emitting diode incorporates some mechanism to inhibit lasing but still allow emission of light.
Briefly, the diode structure includes a substrate of a selected conductivity type, a first cladding layer of the same conductivity type, a thin active layer, and a second cladding layer of the opposite conductivity type. On top of the second cladding layer is a cap layer, and metallized contacts are applied to the cap layer and to the underside of the substrate. When the device is electrically forward-biased, i.e. a voltage is applied in the direction of current flow most favored by the diode, photons are emitted from the active layer at the junction between the two cladding layers.
In one form of the diode, based on a gallium arsenide material system, the current through the junction is confined to a selected small area by the inclusion of a blocking layer between the substrate and the first cladding layer. The blocking layer is of opposite conductivity type opposite to that of the substrate and covers the substrate except for a small area selected for current flow. The voltage applied to the device forward-biases the principal diode junction, but reverse-biases the junction between the substrate and the blocking layer. Therefore, current is inhibited from flowing through the blocking layer and is substantially confined to the selected area in which the substrate is not masked by the blocking layer. The metallization on the cap layer has an opening or window through which light is emitted from the device, and the opening is preferably coated with an anti-reflection coating. The effect of this current confinement technique is to improve the performance of the device by increasing the efficiency, power output, and power-bandwidth product.
In the surface-emitting, current-confinement type of light-emitting diode described, it is preferable to emit a circular pattern of light. A circular pattern is especially more convenient when the output of the diode is to be coupled to an optical fiber. However, it is virtually impossible to obtain a circular emission pattern from a device of this type using a material system based on indium phosphide. This property of indium phosphide devices is unfortunate because the characteristic wavelengths emitted from diodes using an indium phosphide material system are very well suited for use in communications over optical fibers. Circular emission patterns can be obtained from diodes fabricated using a gallium arsenide material system, but the wavelength emitted from these devices is far less desirable for communications use, since losses and dispersion are much greater for the gallium arsenide material system.
The principal difficulty with diodes of this type based on an indium phosphide material system is that, because of the inherent crystalline structure of indium phosphide, it is virtually impossible to etch a circular hole in the material by conventional etching methods, to define the current confinement area. Since indium phosphide is anisotropic in nature, the etched area inevitably becomes elliptical in shape, and the resulting light emission pattern is unacceptable for some applications of the diode. An equally important consideration is that, after etching an opening in the indium phosphide blocking layer, the growth of the first cladding layer is difficult to achieve reproducibly, and the layer may even become "shorted out" electrically to the substrate, resulting in no emission of light. Although special etching procedures may be employed to obtain a circular opening in indium phosphide, the circular shape will, in any event, be modified during the subsequent step of growing the first cladding layer.
It will be appreciated from the foregoing that there is still a need in this area for a technique that avoids these difficulties and provides a current-confinement light-emitting diode with a more symmetrical light emission pattern, even when the material system is indium phosphide. As will now be summarized, the present invention satisfies this need.